The advantages with lignin separation from black liquor is already described in WO 2006/031175 and WO2006/038863. These patents disclose the novel process LignoBoost™ that is now sold by Metso, and wherein WO 2006/031175 discloses the basic two stage acidic wash process and WO2006/038863 disclose an improvement of the process where sulphate or sulphate ions are added to the process.
An important aspect of the process is that the required charge of chemicals for the acidification may be high. If this is the case the cost of fresh chemicals is a large part of the operational cost and the commercial viability of the process is lower. These problems could be reduced, if the process is optimized for minimum requirement for charges of fresh chemicals, making the lignin product commercially sound. Acidifiers in form of mill generated waste flows are thus preferable as it may solve a waste disposal problem and lessen environmental impact. As the precipitation of lignin requires acidification of alkaline black liquor flows, much of the total amount of acidifier is used to decrease the pH level down to the point of where lignin starts to precipitate. The first phase reaching this pH level typically reduce the pH level from about pH 13 in the original black liquor down to a pH level about 11.5, and normally do not involve any nucleation of lignin particles. The amount of acidifier needed is nevertheless relatively high for this first phase as the pH follows a logarithmic scale, and any following additional lowering of pH from 11.5 requires far less acidifier for the same order of lowered absolute pH value.
The Lignoboost process produce a lignin product which if used as fuel is classified as a “green” fuel as being based upon recovered fuel. The idea with classification of “green” fuels is based upon the concept not to increase the carbon dioxide footprint, i.e. the emissions, by burning fossil fuels. The most promising acids for this process is carbon dioxide for at least initial precipitation of the lignin, and then using sulfuric acid (H2SO4) for washing and leaching out metals from the lignin. The sulfuric acid could be added as a fresh sulfuric acid from a chemical supplier, or as preferred using so called “spent acid” from a chlorine dioxide generator often used at a pulp mill. The latter usage of this spent acid already at hand in most mill sites further emphasize that the lignin product is considered as a “green” fuel.
Another problem with the process disclosed in WO 2006/031175 is that there may be a disposal problem with the strongly odorous H2S gases that are emitted from the reslurrying tank and bled out from the process, and it is suggested that these hydrogen sulfides could be added to the pulping process in order to increase sulfidity and possibly increase the yield in the pulping process. However, such rerouting of the strongly odorous H2S gases to another part of the pulp mill introduces risks for emissions of these gases during transport and storage. It is far better to use these gases at the location or process producing these gases.
As the chemical constitution of the original black liquor may change during operation, typically due to changes in the pulping process as of changes in wood material used or cooking conditions, the first precipitation process for precipitating lignin particles from the original black liquor may need adaption to the present conditions. As differing requirements apply for the first precipitation phase, where mainly lowering of pH is the objective, and the second precipitation phase, where lignin starts to precipitate it will be difficult to design a system that meets both these requirements.
The invention is based upon the finding that the precipitation process should be divided into at least two distinctive phases, each adapted for the present phase in the precipitation phase and having its own supply of acidifier.
Thus, the invention is related to a method for separation of lignin from original black liquor having a first pH value, comprising the following phases in sequence:
a first precipitation phase wherein a first acidifier charge is added to the original black liquor in order to decrease the pH value of the original black liquor to a second pH level whereby less than 10% of the total lignin content is precipitated and preferably as small nucleus particles, said second pH level being at least 1 pH unit below that of the first pH value,a second precipitation phase wherein a second acidifier charge is added to the acidified original black liquor from the first precipitation phase in order to decrease the pH value to a third pH level whereby more than 20% of the total lignin content is additionally precipitated and preferably as growth of nucleus particles formed in the first precipitation phase and growth of nucleus particles precipitated in the second precipitation phase, said third pH level being at least 0.1 pH units below that of the second pH value,followed by a separation phase wherein the precipitated lignin is separated from the remaining liquid phase of the acidified original black liquor.
By this method could precipitation be adapted for each individual phase with its individual charge of acidifier, charged in order to meet the objective of each phase.
Preferably is at least 30% of the total lignin content in original black liquor (BLIN) precipitated in total after the second precipitation phase, and that the pH level of the acidified original black liquor is still alkaline, i.e. has a pH level above 7.0 and preferably above 10, after the second precipitation phase. By this embodiment could a part of the total lignin content, typically about 50%, be extracted from the original black liquor, still keeping a part of the heat value of the treated black liquor for any subsequent combustion in a recovery boiler, and the remaining liquid part of the original black liquor could be mixed back into the major part of the original black liquor not causing any problems associated with mixing of acidic waste flows to black liquor.
According to one preferred embodiment is also at least one of the first or second acidifier charges comprising acidifying gas. I.e. liquid acidifier could also be used, but acidic waste gases are often available at a pulp mill and a potential environmental pollution if not destructed in expensive waste gas cleaning systems. It is thus preferable to use these gases as acidifiers in the inventive method. Preferably the acidifying gas is rich in carbon dioxide, and may have its origin from flue gases vented from a lime kiln which naturally contains large amounts of carbon dioxide.
As the inventive method includes at least two distinct phases using acidifying gas charged could at least a part of the flow path of the first acidifying gas led through the first precipitation phase have a random flow path constantly changing flow direction at no straight flow path longer than 5 centimeter, preferably less than 1 centimeter, said flow path created by random packing of filling bodies in said flow path. Such a routing of the gases through the flow of black liquor increase the dissolving capacity of the acidifying gas and hence obtain a same pH in said phase with less charge of acidifier gas or lower pH with similar charge. The filling bodies used could preferably be of a type similar to Rachig-rings normally used in gas contacting columns or filters, or other shape of irregular filling bodies.
In a further embodiment could also at least a part of the flow path of the original black liquor from the first precipitation phase led through the second precipitation phase have an open flow path allowing a straight flow path longer than 5 centimeter, with flow restrictions allowing precipitated lignin to move with the flow of the black liquor with a flow deflection of the precipitated lignin being less than 80 degrees in relation to the general flow direction of the black liquor through the second precipitation phase, hence allowing any precipitated lignin particles flow with at least one flow vector being parallel to the general flow. By this design could be avoided that precipitated lignin may block the flow path of the black liquor and totally stop the process.
In yet a further embodiment of the inventive method using acidifier gas is the original black liquor flowing downwards in the first precipitation phase wherein a first acidifier gas is led countercurrent to flow of original black liquor. This embodiment may enable longer retention time of the acidifier in the flow of black liquor, and increase the dissolving capacity of the acidifying gas.
As an alternative embodiment of the inventive method is the original black liquor flowing upwards in the first precipitation phase wherein a first acidifier gas is led concurrent with flow of original black liquor. This may be preferable if a lower concentration of acidifier gas is needed in the position where precipitation of lignin nucleus particle may start, as high concentration of acidifier gas may result in excessive formation of small nucleus particles instead of lignin particle growth.
Most of the acidifier needed for acidification and precipitation of the lignin from the black liquor could be obtained from flue gases vented from a lime kiln at the mill site. Typically the content of carbon dioxide in these flues gases is well above 25%. By using these flue gases for acidification and precipitation would emissions from the lime kiln in aspects of carbon dioxide be reduced significantly, and no fresh carbon dioxide needs to be added to the Lignoboost process. Only by using the flue gases from the lime kiln could the pH of the black liquor be lowered by 1.5 to 2.5 units, i.e. from an original pH level above pH 13 down to a pH level in the order of 11.5, thus only initiating a smaller first precipitate fraction of lignin from the original black liquor mostly containing small lignin nucleus particles
In yet a preferred embodiment of the inventive method is at least a part of the flue gases vented from the lime kiln first used for dewatering the lignin cake before being used as acidifier in the first precipitation phase. This improves the dewatering of the lignin product as well as takes care of any environmental problems with dust emissions from the dewatering phase. The dust would then be brought into the precipitation phase and collected in the lignin product precipitated.
In a further preferred embodiment of the invention are also further carbon dioxide and H2S gases emitted from second acidification phase in the Lignoboost process re circulated and mixed with the original black liquor in the first precipitation phase. By using this re-circulation could almost the entire need for added acidifier in the precipitation phase be fulfilled by using only lime kiln flue gases and internal gases from the process. If the Lignoboost process is implemented to precipitate lignin from a semi-evaporated original black liquor having a concentration of solids of about 42%, could as much as 9.6 ton of lignin per hour be precipitated from a black liquor flow of about 103 m3/h.
The H2S gases that are emitted from the reslurrying tank in the Lignoboost process contain a large amount of residual carbon dioxide, CO2. By re-circulating this H2S and CO2 rich gas back to the first acidification phase a corresponding reduction of addition of the fresh carbon dioxide is obtained. Only by using the flue gases from the lime kiln in a first phase could the pH of the black liquor be lowered by 1.5 to 2.5 units, i.e. from an original pH level above pH 13 down to a pH level in the order of 11.5, thus initiating a first precipitate fraction of lignin from the original black liquor, and re circulation of the H2S gases emitted from second acidification phase to the precipitation phase could lower the pH further down from 11.5 down to a pH level in the order of 11.2, thus initiating a second larger precipitate fraction of lignin from the original black liquor.
As indicated above could the precipitation stage be implemented in first and second phases which either could be implemented in one and the same vessel or in two separate vessels. When the precipitation stage comprises two separate precipitation phases, treating the original black liquor in series, could at least a part of the gases rich in carbon dioxide and having its origin from flue gases vented from a lime kiln be added to the first phase of the first precipitation stage. As the lime kiln flue gases comes in great volumes could this absorption process for the carbon dioxide content be optimized for these large gas volumes.
If the precipitation stage comprises two separate precipitation phases, then the waste gases emitted from the second acidification stage could be re circulated and mixed with the original black liquor in the second phase of the first precipitation stage.
The carbon dioxide formed in the reslurrying tank, originates from the sulphides and carbonates content in the lignin cake. These compounds react with the acidifier and forms carbon dioxide (CO2) and hydrogen sulfide (H2S), according to:CO32−+2H+<->CO2+H2OHCO3−+H+<->CO2+H2OS2−+2H+<->H2SHS−+H+<->H2S
The formation of carbon dioxide in this process enables a further source for carbon dioxide needed for the first acidification phase, and the hydrogen sulfide is also a net contributor to the acidification as the pKa value of hydrogen sulfide is 6.89.
In a further preferred embodiment of the inventive method are the flue gases vented from a lime kiln first used for dewatering the lignin cake or lignin product in at least one of the first, second and/or third dewatering stages before being used as acidifier in the precipitation stage. This usage of the hot flue gases as means for dewatering the lignin cake or lignin product could in one or several positions of the Lignoboost process be implemented in parallel or preferably in series by sending the flue gases countercurrent to the lignin flow through the process.
In a further preferred embodiment of the inventive method is the entire process, from the second acidification stage, i.e. excluding the first precipitation stage which is kept alkaline, and until obtaining the final lignin product, kept at acidic conditions below pH 6. Preferably the entire process from the second acidification stage is kept at acidic conditions even below pH 4. The pH level throughout the process is most preferred at a pH from 1 to 3.5. This would prevent any separated lignin from being dissolved again, and the precipitated lignin would be subjected to repeat leaching of metals and other unwanted components, meeting the objectives of obtaining a clean lignin product at high yield.
The inventive method may also include the additional steps of combining the pH level adjustment with an adjustment of the ion strength, preferably by using alkali metal ions or alkaline earth metal ions, most preferred calcium ions.
It is intended throughout the present description that the expression “dewatering” embraces any means of dewatering. Preferably the dewatering is performed by using centrifugation, a filter press apparatus, a band filter, a rotary filter, such as a drum filter, or a sedimentation tank, or similar equipment, most preferred a filter press apparatus is used.
It is intended throughout the present description that the expression “original black liquor” embraces spent cooking liquor from a digester, having most of the lignin from the original cellulose material dissolved in the “original black liquor”. The “original black liquor” may also have a large content of organic and inorganic material, but may also have passed through separation processes for extracting turpentine or other specific constituents, while keeping the bulk volume of dissolved lignin unaltered.
It is intended throughout the present description that the expression “lime kiln” embraces the conversion plant in the recovery island where the calcium carbonate in the lime mud obtained in the recaustizising plant is calcined to calcium oxide and reused in the lime cycle.